1. Field of the Invention
This invention relates to a driving circuit of an ultrasonic wave (vibration wave) motor in which electro-mechanical energy converting elements such as electrostrictive elements, magnetostrictive elements or piezoelectric elements are used to generate a travelling vibration wave and a rotor is driven by the vibration wave.
2. Related Background Art
In an ultrasonic wave motor, as shown in FIG. 1 of the accompanying drawings, an electrostrictive element polarization-treated in a conventional phase relation is disposed on the surface of a ring-shaped stator, and on the electrostrictive element, driving periodic voltages which are 90.degree. out of phase with each other are applied to driving electrodes 1-1 and 1-2 to generate a travelling vibration wave in the surface of the stator and a moving member which is in frictional contact with the stator is moved by the vibration wave.
When voltages are to be applied to the driving electrodes 1-1 and 1-2, periodic voltages which are 90.degree. out of phase with each other are applied through an amplifier 7, coil 10, electrode 1-1 and an amplifier 8, coil 11, electrode 1-2, as shown in FIG. 2 of the accompanying drawings.
As regards the phase relation between the amplifier 7, the coil 10 and the electrode 1-1, considering an electrical equivalent circuit between the driving electrode 1-1 and a common electrode 1-4 (including an electrostrictive element), the equivalent circuit is represented as a series circuit of a resistor and a capacitor and therefore, the output voltage of the amplifier 7 and the signal at the junction between the coil 10 and the electrode 1-1 become different in phase.
The above-described phase relation also holds true of the phase difference relation between the output of the amplifier 8 and the signal at the junction between the coil 11 and the electrode 1-2.
Consequently, if the electrical characteristics of the coil 10 and the coil 11 and of the electrode 1-1 and the electrode 1-2 are just the same, even if there is a phase difference between the junction between the coil and the electrode and the output of the amplifier, the phase relation between the junction between the electrode 1-1 and the coil 10 and the junction between the electrode 1-2 and the coil 11 is just the same as the phase relation between the amplifiers 7 and 8 and therefore, no problem will arise if the phase difference between the outputs of the amplifiers 7 and 8 is 90.degree. .
However, it is difficult to make the characteristics of the coils 10 and 11 and of the electrodes 1-1 and 1-2 entirely identical and it is also difficult to make the characteristics of the amplifiers 7 and 8 entirely identical and therefore, even if the input signals to the amplifiers 7 and 8 are signals which are 90.degree. out of phase with each other, it is difficult to ensure the phase characteristics of the amplifiers 7 and 8 to be accurately 90.degree. out of phase.
Also, there is no assurance that the relation between the moving member and the stator 1 is entirely uniform due to the angle of rotation thereof.
Accordingly, even if the formed driving signals have a 90.degree. phase difference relation, it has been impossible that the signals actually applied to the electrodes 1-1 and 1-2 are 90.degree. out of phase with each other.